Carbon-Carbon Bond Formation by Radical Cyclisation: Regioselective Synthesis of Coumarin-Annulated Sulfur Heterocycles

Abstract

A simple convergent synthesis of cis-benzothiopyrano[3,2-c]benzopyran-7(2H)-ones 4a-f (70-75%) through implementation of a regioselective ‘6-endo-trig’ aryl radical cyclisation of the respective 4-[(2-bromobenzyl)sulfanyl]-2H-chromen-2-ones 3a-f (80-85%) with tributyltin hydride in the presence of a radical initiator (AIBN) is described. The starting materials 3a-f are synthesised by phase-transfer-catalysed reaction of 4-sulfanyl-2H-chromen-2-ones 1a-c with 2-bromobenzyl bromides 2a,b.

Key words

heterocyclic compounds - sulfur heterocycles - organotin reagent - 4-sulfanylcoumarin - radical cyclisation - 6-endo-trig

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